This invention relates to method of fabricating a channel member for a plasma addressed liquid crystal (PALC) display panel.
U.S. Pat. No. 5,077,553 discloses apparatus for addressing data storage elements. A practical implementation of the apparatus shown in U.S. Pat. No. 5,077,553 is illustrated schematically in FIG. 4 of the accompanying drawings.
The display panel shown in FIG. 4 comprises, in sequence from below, a polarizer 2, a channel member 4, a cover sheet 6 (commonly known as a microsheet), a layer 10 of electro-optic material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 4), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. The channel member 4 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20, which are separated by ribs 22, are filled with an ionizable gas, such as helium. An anode 24 and a cathode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 28. Each panel element can be considered to include elements of the layer 10 and the lower and upper polarizers 2 and 16. In the case of a color display panel, the panel elements include color filters (not shown) between the layer 10 and the upper substrate 14. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 30 of the display panel.
When the anode in one of the channels is connected to ground and a suitable negative voltage is applied to the cathode in that channel, the gas in the channel forms a plasma that provides a conductive path at the lower surface of the cover sheet 6. If a data drive electrode is at ground potential, there is no significant electric field in the volume element of electro-optic material in the panel element at the crossing of the channel and the data drive electrode and the panel element is considered to be off, whereas if the data drive electrode is at a substantially different potential from ground, there is a substantial electric field in that volume element of electro-optic material and the panel element is considered to be on.
It will be assumed in the following description, without intending to limit the scope of the claims, that the lower polarizer 2 is a linear polarizer and that its, plane of polarization can be arbitrarily designated as being at 0.degree. relative to a reference plane, that the upper polarizer 16 is a linear polarizer having its plane of polarization at 90.degree., and that the electro-optic material is a twisted nematic liquid crystal material that rotates the plane of polarization of linearly polarized light passing therethrough by an angle that is a function of the electric field in the liquid crystal material. When the panel element is off, the angle of rotation is 90.degree.; and when the panel element is on, the angle of rotation is zero.
The panel is illuminated from the underside by an extended light source (not shown) that emits unpolarized white light. A rear glass diffuser 18 having a scattering surface may be positioned between the light source and the panel in order to provide uniform illumination of the panel. The light that enters a given panel element from the source is linearly polarized at 0.degree. by the lower polarizer 2 and passes sequentially through the channel member 4, the channel 20, the cover sheet 6, and the volume element of the liquid crystal material toward the upper polarizer 16 and a viewer 32. If the panel element is off, the plane of polarization of linearly polarized light passing through the volume element of liquid crystal material is rotated through 90.degree., and therefore the plane of polarization of light incident on the upper polarizer element is at 90.degree.. The light is passed by the upper polarizer element and the pixel is illuminated. If, on the other hand, the panel element is on, the plane of polarization of the linearly polarized light is not changed on passing through the volume element of liquid crystal material. The plane of polarization of light incident on the upper polarizer element is at 0.degree. and therefore the light is blocked by the upper polarizer element and the pixel is dark. If the electric field in the volume element of liquid crystal material is intermediate the values associated with the panel element being off and on, light is passed by the upper polarizer element with an intensity that depends on the electric field, allowing a gray scale to be displayed.
There are three principal methods currently used for fabricating the channel member of a PALC display panel. In accordance with one method, a glass substrate is etched to form an array of parallel channels in its upper surface and the anodes and cathodes are then formed in the channels.
Another method involves depositing layers of paste on discrete areas of a glass substrate using a screen printing process.
The third method involves depositing blanket layers of material on a glass substrate and selectively removing the material by sandblasting.
The channel member 4 shown in FIG. 4 is formed using the first method described above. For ease of processing, the anodes of the channel member shown in FIG. 4 are formed at the same time as the cathodes and are of the same structure as the cathodes.
During operation of the PALC panel, the cathode is subject to sputtering by the positive ions of the plasma. It has been proposed that the cathode should be protected from sputtering damage by providing a protective top coating of refractory material over the cathode. See U.S. Pat. No. 5,783,906 and U.S. Provisional Patent Application No. 60/023,418 (Attorney Docket No. 5843-US-1). The coating of refractory material may be composed of a rare earth hexaboride, such as LaB.sub.6, in which case the coating may be deposited by cataphoretic deposition.
It has also been suggested that it may be advantageous if the rare earth hexaboride is not deposited on the anodes and that the anodes should have a surface layer that is homogeneous and is of high electrical conductivity. See U.S. Provisional Patent Application No. 60/026,661 (Attorney Docket 6279-US-0), the disclosure of which is hereby incorporated by reference herein.
In accordance with the disclosure in U.S. Provisional Patent Application No. 60/026,661 (Attorney Docket 6279-US-0), it is proposed that the channel member should be placed in an electrophoresis deposition cell and that the cathodes be connected to the negative terminal of the cataphoretic deposition voltage source but that the anodes should remain out of circuit or be connected to a terminal at a positive voltage relative to the negative terminal of the cataphoretic deposition voltage source. The spacing between the anode and cathode in a channel of the channel member may be as small as 100 .mu.m, and even though most of the rare earth hexaboride will be deposited on the cathodes, it is still possible that a significant quantity of the rare earth hexaboride will be deposited on the anodes.